Revisiting Chemically Enhanced Primary Treatment of Wastewater: A Review

Application of electrospun N-doped carbon dots loaded cellulose acetate membranes as cationic dyes adsorbent

This work aims to apply carbon quantum dots (CQDs) from agriculture cellulosic waste (agro wastes), produced via an economically and eco-friendly single-step method, to be used into cellulose acetate composite microfibrous membranes as an innovative solution specifically designed to adsorb methylene blue (MB) and other cationic dyes that are present in various water effluents. Batch adsorption tests were conducted, with variations in contact time (1-24 h), initial MB concentration (25-300 ppm), and adsorbent doses (1-20 g/L). The maximum adsorption capacity of the membrane was 198 mg/g with an initial concentration of 300 ppm at 298 K. Thermodynamic parameters showed that the process is endothermic. Equilibrium experimental data for MB adsorption onto electrospun adsorbent were fitted using different isothermal models, with the Freundlich model showing the best fit. The pseudo-second-order model accurately described the kinetic data with high reliability (R2 > 0.99), and the calculated adsorption capacity was very close to the experimental data. N-CQDs loaded membranes were also tested for removing methyl violet and rhodamine B, demonstrating remarkably high dye removal efficiency. The underlying adsorption mechanism was also reported. Finally, it is worth mentioning that composite adsorbents can be efficiently applied to actual industrial cases because of the possibility of reusing them, opening the route to the fabrication of novel and highly performant adsorbents. These findings underscore N-CQDs' effectiveness in enhancing pollutant removal efficiency from wastewater, highlighting their environmental benefits and promoting a more sustainable approach to water treatment. Therefore, the prepared adsorbent, showing excellent adsorption performance, places them among adsorbents for practical applications in wastewater purification.

Iron driven organic carbon capture, pretreatment, recovery and upgrade in wastewater: Process technologies, mechanisms, and implications

Wastewater treatment plants face significant challenges in transitioning from energy-intensive systems to carbon-neutral, energy-saving systems, and a large amount of chemical energy in wastewater remains untapped. Iron is widely used in modern wastewater treatment. Research shows that leveraging the coupled redox relationship of iron and carbon can redirect this energy (in the form of carbon) towards resource utilization. Therefore, re-examining the application of iron in existing wastewater carbon processes is particularly important. In this review, we investigate the latest research progress on iron for wastewater carbon flow restructuring. During the iron-based chemically enhanced primary treatment (CEPT) process, organic carbon is captured into sludge and its bioavailability is enhanced through iron-based advanced oxidation processes (AOP) pretreatment, further being recovered or upgraded to value-added products in anaerobic biological processes. We discuss the roles and mechanisms of iron in CEPT, AOP, anaerobic biological processes, and biorefining in driving organic carbon conversion. The dosage of iron, as a critical parameter, significantly affects the recovery and utilization of sludge carbon resources, particularly by promoting effective electron transfer. We propose a pathway for beneficial conversion of wastewater organic carbon driven by iron and analyze the benefits of the main products in detail. Through this review, we hope to provide new insights into the application of iron chemicals and current wastewater treatment models.

Combined high-rate contact stabilization and chemically enhanced primary treatment for enhanced recovery of organic matter and biogas from sewage

This study examined integrating high-rate contact stabilization (HRCS) and chemically enhanced primary treatment (CEPT) for wastewater to improve the carbon recovery rate (CRR). Enhancing chemical oxygen demand (COD) removal efficiency was hypothesized to improve the CRR. The evaluation covered serial HRCS-CEPT, serial CEPT-HRCS, and single-stage carbon recovery (Single-CR). The COD removal efficiencies for individual HRCS and CEPT were 50.3 % and 56.2 %, respectively. The serial CEPT-HRCS system failed in the HRCS process due to poor settling, resulting in microbial washout. However, the serial HRCS-CEPT system achieved the highest COD removal efficiency (84.5 %). The Single-CR system exhibited the highest CRR of 0.780 ± 0.083 g-CODCH4/g-CODinf, identifying it as the most promising process for energy-positive wastewater treatment. The selective pressure in the high-rate system resulted in a simplified and specialized bacterial community, mainly comprising microorganisms with high polyhydroxyalkanoate storage capacity, such as Lactococcus sp., Enterobacter sp., and Acinetobacter sp.

Densification of Synechococcus subsalsus biomass by chitosan coagulation for biogas production

This study evaluated the coagulation/flocculation process using chitosan as a natural coagulant to concentrate suspensions of the cyanobacterium Synechococcus subsalsus and enable biogas production from concentrated biomass. The chitosan performance was tested and compared with the inorganic ferric chloride (FeCl3) coagulant. Using the liquid fraction of the coagulation/flocculation process in subsequent biomass cultivations proved viable, with similar growths in culture media with up to 80% of the liquid fraction. At pH 6 and 400 mg/L FeCl3, the biomass concentrated almost seven times, increasing the total suspended solids (TSS) of the suspension from 0.4-0.6 g/L to 2.6-4.0 g/L. With 80 mg/L chitosan and pH 7, the TSS concentration attained values in the range of 7.0-9.7 g/L, an increase of more than 30 times, clearly showing that chitosan has a much higher capacity for biomass concentration at a lower concentration. A ratio of 0.3 g chitosan/g dry mass of the biomass was established to reach the maximum densification. The production of methane from chitosan-densified biomass proved to be feasible. Chitosan-densified biomass showed a two-phase cumulative methane production when digested, with slower methane production and 23% lower methane yield after 30 days of digestion (207 NmL CH4/g CODi) compared to the biomass from cultivation (non-densified, 270 NmL CH4/g CODi). However, optimizing the digestion conditions of the densified biomass should increase the methane yield and reduce process time.

Iron Recycle-Driven Organic Capture and Sidestream Anaerobic Membrane Bioreactor for Revolutionizing Bioenergy Generation in Municipal Wastewater Treatment

The practicality of intensifying organic matter capture for bioenergy recovery to achieve energy-neutral municipal wastewater treatment is hindered by the lack of sustainable methods. This study developed innovative processes integrating iron recycle-driven organic capture with a sidestream anaerobic membrane bioreactor (AnMBR). Iron-assisted chemically enhanced primary treatment achieved elemental redirection with 75.2% of chemical oxygen demand (COD), 20.2% of nitrogen, and 97.4% of phosphorus captured into the sidestream process as iron-enhanced primary sludge (Fe-PS). A stable and efficient biomethanation of Fe-PS was obtained in AnMBR with a high methane yield of 224 mL/g COD. Consequently, 64.1% of the COD in Fe-PS and 48.2% of the COD in municipal wastewater were converted into bioenergy. The acidification of anaerobically digested sludge at pH = 2 achieved a high iron release efficiency of 96.1% and a sludge reduction of 29.3% in total suspended solids. Ultimately, 87.4% of iron was recycled for coagulant reuse, resulting in a theoretical 70% reduction in chemical costs. The novel system evaluation exhibited a 75.2% improvement in bioenergy recovery and an 83.3% enhancement in net energy compared to the conventional system (primary sedimentation and anaerobic digestion). This self-reliant and novel process can be applied in municipal wastewater treatment to advance energy neutrality at a lower cost.

Enhanced nitrogen removal via partial nitrification/denitrification coupled Anammox using three stage anoxic/oxic biofilm process with intermittent aeration

Pre-capturing organics in municipal wastewater for biogas production, combined with Anammox-based nitrogen removal process, improves the sustainability of sewage treatment. Thus, enhancing nitrogen removal via Anammox in mainstream wastewater treatment becomes very crucial. In present study, a three-stage anoxic/oxic (AO) biofilm process with intermittent aeration was designed to strengthen partial nitrification/denitrification coupling Anammox (PNA/PDA) in treatment of low C/N wastewater, which contained chemical oxygen demand (COD) of 79.8 mg/L and total inorganic nitrogen (TIN) of 58.9 mg/L. With a hydraulic retention time of 8.0 h, the process successfully reduced TIN to 10.6 mg/L, achieving a nitrogen removal efficiency of 83.3 %. The 1st anoxic zone accounted for 32.0 % TIN removal, with 10.3 % by denitrification and 21.7 % by PDA, meanwhile, the 2nd and 3rd anoxic zones contributed 19.4 % and 4.5 % of TIN removal, primarily achieved through PDA (including endogenous PD coupling Anammox). The 1st and 2nd intermittent zones accounted for 27.2 % and 17.0 % of TIN removal, respectively, with 13.7 %-21.3 % by PNA and 3.2 %-5.3 % by PDA. Although this process did not pursue nitrite accumulation in any zone (< 1.5 mg-N/L), PNA and PDA accounted for 35.1 % and 52.1 % of TIN removal, respectively. Only 0.21 % of removed TIN was released as nitrous oxide. The AnAOB of Candidatus Brocadia was enriched in each zone, with a relative abundance of 0.66 %-2.29 %. In intermittent zones, NOB had been partially suppressed (AOB/NOB = 0.73-0.88), mainly due to intermittent aeration and effective nitrite utilization by AnAOB since its population size was much greater than NOB. Present study indicated that the three-stage AO biofilm process with intermittent aeration could enhance nitrogen removal via PNA and PDA with a low N2O emission factor.

Reduction of Malnutrition Related to Unsafe Water Consumption in Developing Countries: Potabilization of Surface Water and Traditional Well Water, with Plant Extracts

The consumption of unsafe water in rural areas is a real public health problem in developing countries. This situation mainly affects children under five years of age and causes several deaths and many cases of malnutrition every year. The objective of this study was to evaluate and optimize the capacity of four local plant extracts in the potabilization of unsafe water. Thus, Moringa oleifera and Boscia senegalensis seeds, or Aloe vera and Opuntia ficus-indica mucilages were prepared in a solution and applied during a jar test as biocoagulants and bioflocculants on three raw water samples of 82.3 NTU, 549.8 NTU and 796.9 NTU. After treatment results showed that 0.9 g/L of Moringa biocoagulant or 1 g/L of Boscia biocoagulant applied with 0.4 mL of Aloe vera bioflocculant or 0.6 mL of Opuntia ficus-indica bioflocculant reduced the turbidity of each water sample to values less than 5 NTU after only 15 min of decanting. Moreover, the sanitary quality of the water treated by these different extracts showed a perfect conformity of the physicochemical and microbiological parameters with the standards of acceptability in drinking water decreed by the World Health Organization. Thus, the application of these local plant extracts has made it possible to considerably improve the quality of unsafe water in record time. Their popularization could be an alternative in the fight against malnutrition related to the consumption of unsafe water, especially in rural areas.

Bioaccumulation of pollutants in the green-lipped mussel Perna viridis: Assessing pollution abatement in Victoria Harbour and its adjacent aquaculture area, Hong Kong, and the minimal human health risks from mussel consumption

The green-lipped mussel Perna viridis was utilised for pollution biomonitoring in Victoria Harbour and its adjacent aquaculture area in Hong Kong. P. viridis was collected from a reference site and redeployed at five study sites for five weeks during the dry and wet seasons of 2019. Our study found various polycyclic aromatic hydrocarbons (PAHs) and heavy metals in the mussel tissue, while polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) were not detected. P. viridis at the reference site generally displayed lower levels of pollutants. Comparing with previous research in the 1980s and 2000s, we observed substantial reduction in the tissue levels of PAHs, PCBs, OCPs and heavy metals in P. viridis. The human health risks associated with consuming these mussels were determined to be insignificant. Our findings imply that the Harbour Area Treatment Scheme has been effective in improving the water quality in Victoria Harbour and its adjacent aquaculture area.

Excellent coagulation performance of polysilicate aluminum ferric for treating oily wastewater from Daqing gasfield: Responses to polymer properties and coagulation mechanism

The polysilicate aluminum ferric (PSAF) was synthesized via copolymerization of polysilicic acid (PSi), AlCl3 and FeCl3 for treating oily wastewater from Daqing gas field. This study investigated the effects of key preparation factors such as the degree of PSi's preactivation and the ratio of (Fe + Al)/Si and Al/Fe on both polymerization and coagulation performance exhibited by PSAF. To determine the optimal timing for introducing Al3+ and Fe3+, zeta potential, viscosity and particle size were investigated. Additionally, infrared spectroscopy, X-ray powder diffraction, polarizing microscopy and scanning electron microscope analysis were employed to investigate the structure and morphology of PSAF. The results indicate that under conditions characterized by a SiO2 mass fraction of 2.5% and pH = 4.5, an optimal timing for introducing Al3+ and Fe3+ is at 100 min when PSi exhibits moderate polymerization along with sufficient stability. When considering molar ratios such as (Al + Fe)/Si being 6:4 and Al/Fe being 5:5, respectively, PSAF falls within a "stable zone" enabling storage period up to 32 days. Moreover, Jar test results demonstrate that at a dosage of 200 mg/L PSAF for oily wastewater treatment in gas fields could reach the maximum turbidity removal efficiency up to 99.5% while oil removal efficiency reach 88.6% without pH adjustment. The copolymerization facilitates the formation of larger PSAF aggregates with positive potential, thereby augmenting the coagulants' adsorption bridging and charge neutralization capabilities. As a result, PSAF has great potential as a practical coagulant for treating oil-containing wastewater in industrial settings.

Carbon redirection in chemically enhanced primary treatment of domestic wastewater: A meta-analysis of laboratory to full-scale trials

Increasing energy demands combined with local scarcities and rising prices make the valorisation of energy from domestic wastewater seen as a valuable resource. Chemically enhanced primary treatment (CEPT) enables an increased redirection of organic compounds into sludge in the primary stage of a wastewater treatment for a transformation into biogas (carbon capture). Traditionally used coagulants consist of metallic salts, but in the last two decades, the development of polymers, based on petroleum or synthesized from renewable sources such as plants, has been intensified. However, a direct comparison of the effectiveness of these products is missing. In this paper, we analysed data of peer-reviewed research from jar tests to full-scale studies, highlighting key parameters for successful carbon capture. More than 100 studies were identified, with a majority presenting results from tests under static conditions (jar tests), while data on full-scale applications are scarce. Overall, for TSS and COD, a clear correlation between inflow concentration and removal efficiency was found, irrespective of the product used. Comparison between the effectiveness of the different types of products is difficult, but bio-based coagulants need to be generally added in higher product concentrations for a considerable removal efficiency. While CEPT seems to increase the general sludge and biogas output, future studies should focus on harmonising laboratory analysis to make results comparable. Another important issue that should be addressed is the provision of experimental details, especially for full-scale trials, to enable for reliable conclusions.

Impact of coagulation on SARS-CoV-2 and PMMoV viral signal in wastewater solids

Wastewater surveillance (WWS) of SARS-CoV-2 has become a crucial tool for monitoring COVID-19 cases and outbreaks. Previous studies have indicated that SARS-CoV-2 RNA measurement from testing solid-rich primary sludge yields better sensitivity compared to testing wastewater influent. Furthermore, measurement of pepper mild mottle virus (PMMoV) signal in wastewater allows for precise normalization of SARS-CoV-2 viral signal based on solid content, enhancing disease prevalence tracking. However, despite the widespread adoption of WWS, a knowledge gap remains regarding the impact of ferric sulfate coagulation, commonly used in enhanced primary clarification, the initial stage of wastewater treatment where solids are sedimented and removed, on SARS-CoV-2 and PMMoV quantification in wastewater-based epidemiology. This study examines the effects of ferric sulfate addition, along with the associated pH reduction, on the measurement of SARS-CoV-2 and PMMoV viral measurements in wastewater primary clarified sludge through jar testing. Results show that the addition of Fe3+ concentrations in the conventional 0 to 60 mg/L range caused no effect on SARS-CoV-2 N1 and N2 gene region measurements in wastewater solids. However, elevated Fe3+ concentrations were shown to be associated with a statistically significant increase in PMMoV viral measurements in wastewater solids, which consequently resulted in the underestimation of PMMoV-normalized SARS-CoV-2 viral signal measurements (N1 and N2 copies/copies of PMMoV). The observed pH reduction from coagulant addition did not contribute to the increased PMMoV measurements, suggesting that this phenomenon arises from the partitioning of PMMoV viral particles into wastewater solids.

Microbiological and physicochemical quality enhancement of treated wastewater using raw and chemically modified clays from Sidi Bouzid region, Tunisia

Environmental discharge of wastewater represents a source of chemical and biological pollutants. This study firstly evaluates the microbiological and physicochemical quality of treated wastewaters collected from two wastewater treatment plants (WWTPs) located in two different Tunisian cities namely Sidi Bouzid (SB) and Gafsa (G). Then, the capacity of three raw and acid/base-activated local clays to enhance the quality of wastewaters was assessed. The results indicate that the quantities of enteric bacteria (oscillating from 1.381 × 103 to 1.4 × 108 CFU/100 mL), fungi (between 1.331 × 103 and 1.781 × 104 CFU/100 mL), as well as SARS-CoV-2 (between 4.25 × 103 and 5.05 × 105 CFU/100 mL) and Hepatitis A virus RNA (form 4.25 × 103 to 7.4 × 104 CFU/100 mL) detected in effluent wastewaters were not in compliance with the Tunisian standards for both studied WWTPs. Likewise for other indicators such as electrical conductivity (ranging 4.9-5.4 mS/cm), suspended matter (145-160 g l-1), chemical oxygen demand (123-160 mg l-1), biological oxygen demand 5 (172-195 mg l-1), chloride, Total Kjeldahl nitrogen (TKN) and phosphorus contents (710, 58-66 and 9.47-10.83 mg l-1 respectively), the registered values do not agree with the set standards established for wastewater treatment. On the other hand, the pH values fitted (oscillating from 6.86 (at G) to 7.24 (at SB) with the Tunisian standards for both WWTPs. After treatment, wastewaters showed better values for the microbiological parameters, especially for the clays designed as AM and HJ1, which eliminated 100% of viruses. In addition, when acid-activated AM clays were applied, a marked improvement in the quality of physicochemical parameters was obtained, especially for suspended matter (2 and 4 g l-1 for SB and G, respectively), TKN (5.2 (SB) and 6.40 (G) mg/l), phosphorus (1.01 (SB) and 0.81 (G) mg/l). Our results open perspectives for the possibility of efficiently using these specific clays in the enhancement of the quality of treated wastewaters.

Process for nutrient recycling from intensive aquaculture through microalgae-bacteria consortium

This work studies a biological process based on a microalgae-bacteria consortium for recycling nutrients in a recirculating aquaculture system (RAS) implanted in an intensive marine aquaculture farm. Additionally, some techniques were used for microalgae biomass harvesting and tested the effectiveness of filtration by a column with multi-layer sand to reduce the solids concentrations in the effluent. The consortium was grown in photobioreactors in batch and semi-continuous operation modes using the solids concentrated stream generated in the RAS system. The semi-continuous operation showed a high percentage of TDN and TDP removal, achieving final concentrations of 1.09 ± 0.02 mg·L-1 and 0.01 ± 0.01 mg·L-1, respectively, while DOC was reduced to 3.87 ± 0.06 mg·L-1. The values of productivity 44 ± 9 mg TSS·L-1 indicated that the studied stream is a suitable culture medium for the growth of the microalgae-bacteria consortium. A combination of harvesting techniques was studied, coagulation-flocculation-settling and coagulation-flocculation-flotation. The first step was to optimise the dose of FeCl3 through the coagulation-flocculation test to pre-concentrate the biomass generated, achieving an optimal dose of 0.106 mg Fe·mg TSS-1. Then, two separation processes were applied to the stream and compared: settling and flotation. The maximum removal efficiency (90.2 ± 0.3 %) was obtained in the settling process, so the coagulation-flocculation-settling was select as the best combination of harvesting techniques. Finally, sand filtration was studied as an effluent refining process to improve solids reduction of the water obtained in the harvesting step resulting in an effluent with 17.18 ± 1.49 mg TSS·L-1. The proposed sequence process is capable of recycling nutrients from an intensive marine aquaculture farm by using these resources via transformation into microalgae biomass and generating quality effluent.

Primary settling changes the microbial community of influent wastewater to wastewater treatment plants

The continuous immigration of bacteria in influent wastewater strongly impacts the microbial community of activated sludge (AS) in wastewater treatment plants (WWTP), both in terms of species composition and their abundance. Therefore, it is of interest to elucidate the route of immigrating bacteria into the biological tanks, including the effect of primary settlers. These are commonly used pretreatment units that can possibly selectively increase or reduce the relative abundance of certain bacteria. Species-level identification of the microbial composition of influent wastewater before and after primary settling was carried out in four full-scale municipal WWTPs biweekly over one year by 16S rRNA gene amplicon sequencing. Overall, 37-49% of incoming COD was removed in the primary settlers. Most genera and species were present in the wastewater to all four plants and the trend of these were investigated across the primary settlers. Approximately 50% of the genera had the same trend across at least three WWTPs. Few genera significantly increased in relative read abundance (3.7%) after settling, while 22.3% showed a significant reduction in relative abundance. We investigated process-critical species in AS, such as known nitrifiers, polyphosphate-accumulating organisms, and filamentous bacteria. Most taxa were affected similarly in all WWTPs including multiple genera involved in bulking in AS. However, some genera, e.g., important polyphosphate-accumulating bacteria, had inconsistent trends across WWTPs, suggesting that the characteristics of the wastewater are important for the trend of some bacteria through primary settling. In all cases, primary settling changed the microbial community of the influent wastewater, posing an obvious candidate for upstream control to optimize the assembly of the microbial communities in activated sludge.

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

A comparative study on polyaluminum chloride (PACl) and Moringa oleifera (MO) chemically enhanced primary treatment (CEPT) in enhanced biogas production: anaerobic digestion performance and the Gompertz model

A comparative study was performed to estimate biogas production from sludge produced by organic and inorganic chemically enhanced primary treatments (CEPTs). To this end, the effects of two coagulants, polyaluminum chloride (PACl) and Moringa oleifera (MO), on CEPT and biogas production in anaerobic digestion were surveyed within an incubation period of 24 days. The optimal dosage and pH of PACl and MO were optimized in terms of sCOD, TSS and VS parameters in the CEPT process. Next, the digestion performance of anaerobic digestion reactors fed with sludge obtained from PACl and MO coagulants at a batch mesophilic reactor (37 ± 1 °C) was surveyed from the biogas production, volatile solid reduction (VSR) and Gompertz model. At the optimal conditions (pH = 7 and dosage = 5 mg L^-1), the removal efficiency of COD, TSS and VS in CEPT assisted with PACL was 63, 81 and 56%, respectively. Moreover, CEPT assisted with MO led to the removal efficiency of COD, TSS and VS until 55, 68 and 25%, respectively. The highest methane yield (0.598 L g_{VS removed}^-1) was obtained in an anaerobic digestion reactor with sludge from the MO coagulant. The anaerobic digestion of CEPT sludge instead of primary sludge resulted in higher sCOD removal efficiency, and 43-50% of sCOD was observed compared with the removal of 32% for the primary sludge. Furthermore, the high coefficient of determination (R²) demonstrated the trustworthy predictive precision of the modified Gompertz model with actual data. The combination of CEPT and anaerobic digestion, especially using natural coagulants, provides a cost-effective and practical way to increase BMP from primary sludge.

Reuse of lime sludge from immediate one-step lime precipitation process as a coagulant (aid) in slaughterhouse wastewater treatment

The circularity of wastewater treatment subproducts is on the worldwide agenda. In this way, this work aims to evaluate alternatives for the reuse of sludge from slaughterhouse wastewater treatment. Wetted sludges produced in the immediate one-step lime precipitation process were applied directly or first calcined, as a coagulant or coagulant aid, in the absence or presence of Ca(OH)₂, to slaughterhouse wastewaters with different characteristics. For the best sludge reuse, successive reuses of the sludge were carried out and the characteristics of treated slaughterhouse wastewater were evaluated after each reuse. Results showed a great similarity between slaughterhouse and treated slaughterhouse wastewaters using wetted and calcined sludges as a coagulant for highly contaminated slaughterhouse wastewater. In addition, a great similarity was also observed between the calcined and the wetted sludges, both as a coagulant aid, for all the slaughterhouse wastewaters tested. However, the latter consumed more hydrated lime, more volume of sludge sedimented, and higher concentrations phosphorus and organic matter in the treated wastewater. Calcined sludge as a coagulant aid guaranteed the best slaughterhouse wastewater quality for almost of the tested paramenters (≥94% for absorbances at 254 nm and 410 nm, E. coli, turbidity, and phosphorus, chemical oxygen demand between 3 and 91%, and total Kjeldahl nitrogen between 3 and 62%) independently of the wastewater characteristics. Calcined sludge as a coagulant aid can be three times reused for the tested parameters and slaughterhouse wastewater characteristics without significantly decreasing the quality. The successive sludge reused saves the hydrated lime dose applied (up to 28.4%) and the sedimented sludge volume (up to 24.7%), and can be a solution to stabilize sludge due to the pH increase (sludge pH = 12).

Redirecting carbon to recover VFA to facilitate biological short-cut nitrogen removal in wastewater treatment: A critical review

Wastewater treatment plants (WWTPs) are facing a great challenge to transition from energy-intensive to carbon-neutral and energy-efficient systems. Biological nutrient removal (BNR) can be severely impacted by carbon limitation, particularly for wastewater with a low carbon-to-nitrogen (C/N) ratio, which can significantly increase the operational costs. Waste activated sludge (WAS) is a valuable byproduct of WWTPs, as it contains high levels of organic matter that can be utilized to improve BNR management by recovering and reusing the fermentative volatile fatty acids (VFAs). This review provides a comprehensive examination of the recovery and reuse of VFAs in wastewater management, with a particular focus on advancing the preferable biological short-cut nitrogen removal process for carbon-insufficient municipal wastewaters. First, the method of carbon redirection for recovering VFAs was reviewed. Carbon could be captured through the two-stage A/B process or via sludge fermentation with different sludge pretreatment and process control strategies to accelerate sludge hydrolysis and inhibit methanogens to enhance VFA production. Second, VFAs can support the metabolism of autotrophic N-cycling microorganisms involved in wastewater treatment, such as AOB, NOB, anammox, and comammox bacteria. However, VFAs can also cause inhibition at high concentrations, leading to the partition of AOB and NOB; and can promote partial denitrification as an efficient carbon source for heterotrophic denitrifiers. Third, the lab- and pilot-scale engineering practices with different configurations (i.e., A²O, SBR, UASB) were summarized that have shown the feasibility of utilizing the fermentate to achieve superior nitrogen removal performance without the need for external carbon addition. Lastly, the future perspectives on leveraging the relationships between mainstream and sidestream, nitrogen and phosphorus, autotrophs and heterotrophs were given for sustainable and efficient BNR management.

Novel Use of a Ferric Salt to Enhance Mainstream Nitrogen Removal from Anaerobically Pretreated Wastewater

This study aims to demonstrate a new technology roadmap to support the ongoing paradigm shift in wastewater management from pollutant removal to resource recovery. This is achieved by developing a novel use of an iron salt (i.e., FeCl₃) in an integrated anaerobic wastewater treatment and mainstream anammox process. FeCl₃ was chosen to be dosed in a proposed sidestream unit rather than in a primary settler or a mainstream reactor. This causes acidification of returned activated sludge and enables stable suppression of nitrite-oxidizing bacterial activity and excess sludge reduction. A laboratory-scale system, which comprised an anaerobic baffled reactor, a continuous-flow anoxic-aerobic (A/O) reactor, and a secondary settler, was designed to treat real domestic wastewater, with the performance of the system comprehensively monitored under a steady-state condition. The experimental assessments showed that the system had good effluent quality, with total nitrogen and phosphorus concentrations of 12.6 ± 1.3 mg N/L and 0.34 ± 0.05 mg P/L, respectively. It efficiently retained phosphorus in excess sludge (0.18 ± 0.03 g P/g dry sludge), suggesting its potential for further recovery. About half of influent organic carbon was recovered in the form of bioenergy (i.e., methane). This together with low energy consumption revealed that the system could produce a net energy of about 0.11 kWh/m³-wastewater, assessed by an energy balance analysis.

Low-carbon nitrogen removal from power plants circulating cooling water and municipal wastewater by partial denitrification-anammox

As a reclaimed water reuse strategy, using treated municipal wastewater as power plants circulating cooling water (PPCCW) generates nitrate-rich wastewater due to evaporation requiring retreatment. An innovative low-carbon nitrogen removal process, partial denitrification-anammox (PD-A), was used in this study. The PPCCW and municipal wastewater pre-treated with 10 mg/L Fe³⁺ were simultaneously subjected to the PD-A process. The results showed that the total nitrogen of effluent less than 10 mg/L, and a removal efficiency of 79.67 ± 3.48% was attained. Unclassified_f_Brocadiaceae was the dominant anammox genus, with an increasing percentage (from 0.42 to 1.27%), laterally indicating the reactor stability. Furthermore, the hydrolytic acidifying bacteria SBR1031 and Bacillus increased substantially after feeding with actual wastewater, and the removal efficiencies of organic material and nitrogen increased, indicating that hydrolytic acidifying bacteria have a synergistic effect with PD-A bacteria. Finally, a novel wastewater treatment process that fully recovers carbon, phosphorus, and water was proposed.

Toward Energy Neutrality: Novel Wastewater Treatment Incorporating Acidophilic Ammonia Oxidation

Chemically enhanced primary treatment (CEPT) followed by partial nitritation and anammox (PN/A) and anaerobic digestion (AD) is a promising roadmap to achieve energy-neutral wastewater treatment. However, the acidification of wastewater caused by ferric hydrolysis in CEPT and how to achieve stable suppression of nitrite-oxidizing bacteria (NOB) in PN/A challenge this paradigm in practice. This study proposes a novel wastewater treatment scheme to overcome these challenges. Results showed that, by dosing FeCl3 at 50 mg Fe/L, the CEPT process removed 61.8% of COD and 90.1% of phosphate and reduced the alkalinity as well. Feeding by low alkalinity wastewater, stable nitrite accumulation was achieved in an aerobic reactor operated at pH 4.35 aided by a novel acid-tolerant ammonium-oxidizing bacteria (AOB), namely, Candidatus Nitrosoglobus. After polishing in a following anoxic reactor (anammox), a satisfactory effluent, containing COD at 41.9 ± 11.2 mg/L, total nitrogen at 5.1 ± 1.8 mg N/L, and phosphate at 0.3 ± 0.2 mg P/L, was achieved. Moreover, the stable performances of this integration were well maintained at an operating temperature of 12 °C, and 10 investigated micropollutants were removed from the wastewater. An energy balance assessment indicated that the integrated system could achieve energy self-sufficiency in domestic wastewater treatment.

Occurrence and stability of PCMX in water environments and its removal by municipal wastewater treatment processes

Para-chloro-meta-xylenol (PCMX) is a synthetic antiseptic used extensively to control the spread of germs and viruses, and as a result, enormous amount of PCMX could be discharged to water environments through drainage. To investigate the extent of PCMX contamination, water samples were collected from rivers and coastal waters in Hong Kong, and PCMX concentrations were determined by a newly developed method using liquid chromatography-tandem mass spectrometry combined with stable isotope-dilution. We discovered widespread PCMX pollution in the water environment. Then, we revealed for the first time that PCMX in wastewater is not effectively removed by chemically enhanced primary treatment (CEPT), one of the wastewater treatment processes used in Hong Kong (∼75% of wastewater) and other megacities around the world. This suggests that the CEPT effluent or the primary treatment effluent is an unintended continuous source of pollution for PCMX in water environments. Finally, we found that PCMX was relatively stable in the water environment and could pose a risk to aquatic organisms. These findings underscore the importance of raising public awareness of the environmental consequences from overuse of PCMX-based disinfectants and the need to reevaluate the various wastewater treatment processes in removing PCMX.

Realizable wastewater treatment process for carbon neutrality and energy sustainability: A review

Despite a quick shift of global goals toward carbon-neutral infrastructure, activated sludge based conventional systems inhibit the Green New Deal. Here, a municipal wastewater treatment plant (MWWTP) for carbon neutrality and energy sustainability is suggested and discussed based on realizable technical aspects. Organics have been recovered using variously enhanced primary treatment techniques, thereby reducing oxygen demand for the oxidation of organics and maximizing biogas production in biological processes. Meanwhile, ammonium in organic-separated wastewater is bio-electrochemically oxidized to N₂ and reduced to H₂ under completely anaerobic conditions, resulting in the minimization of energy requirements and waste sludge production, which are the main problems in activated sludge based conventional processes. The anaerobic digestion process converts concentrated primary sludge to biomethane, and H₂ gas recovered from nitrogen upgrades the biomethane quality by reducing carbon dioxide in biogas. Based on these results, MWWTPs can be simplified and improved with high process and energy efficiencies.

Review of hospital effluents: special emphasis on characterization, impact, and treatment of pollutants and antibiotic resistance

Health care institutions generate large volumes of liquid effluents from specific activities related to healthcare, analysis, and research. Their direct discharge into the environment has various negative effects on aquatic environments and human health, due to their high organic matter charges and the presence of various emerging contaminants such as disinfectants, drugs, bacteria, viruses, and parasites. Moreover, hospital effluents, by carrying antibiotics, contribute to the development of antibiotic-resistant microorganisms in the environment. This resistance has become a global issue that manifests itself variously in different countries, causing the transmission of different infections. In this respect, an effort is provided to protect water resources by current treatment methods that imply physical-chemical processes such as adsorption and advanced oxidation processes, biological processes such as activated sludge and membrane bioreactors and other hybrid techniques. The purpose of this review is to improve the knowledge on the composition and impact of hospital wastewater on man and the environment, highlighting the different treatment techniques appropriate to this type of disposal before discharge into the environment.

Nickel removal from wastewater using electrocoagulation process with zinc electrodes under various operating conditions: performance investigation, mechanism exploration, and cost analysis

Economically feasible approaches are needed for wastewater treatment. Electrocoagulation (EC) is an electrochemical treatment method that removes various pollutants from wastewater. It has grown in popularity over conventional treatment methods, especially in industrial wastewater, due to its high performance and the ability to remove toxic compounds. However, it is crucial to reduce the costs associated with EC for widespread implementation. It is also important to decrease nickel (Ni) concentrations in wastewater to prevent potential health and environmental problems. Therefore, this study investigates Ni removal from synthetic and real wastewater using electrocoagulation. Zinc, as a novel electrode, was used as the sacrificial anode. Several operating conditions were assessed, including current density, initial pH, electrolysis time, and spacing between electrodes. The maximum Ni removal efficiency, after 90 min, reached 99.9% at a current density of 10 mA/cm2 when the pH was 9.2 and the gap distance was 4 cm. The Ni removal rate reached 94.4% and 94.9% at a 2- and 6-cm spacing, respectively, after 90 min. Anode morphology, kinetic modeling, electrical energy consumption, and cost analysis were also investigated. The type of corrosion was uniform, which is easily predicted compared to pitting corrosion. The comparison between chemical coagulation and electrocoagulation was also reported. Experimental results indicated that the maximum Ni removal rates reached 99.89% after 90 min. The optimum spacing between electrodes was 4 cm, and the optimum current density was 10 mA/cm2. Additionally, the kinetic data were best represented through the second-order Lagergren model. The results demonstrated that the electrocoagulation performance was better than that of chemical coagulation for Ni removal. The maximum electrical energy consumption was 23.79 KWh/m3 for Ni removal.

Fouling reduction in nanofiltration membranes in the treatment of municipal sewage - effect of coagulant type used for prior chemically enhanced primary treatment

The limiting factor in wide-scale application of membranes for wastewater treatment is membrane fouling. Coagulation has emerged as an effective technique for fouling control. In this research, municipal wastewater was treated using a two-stage treatment. In stage-1, chemically enhanced primary treatment (CEPT) was rendered using an optimum dose of two coagulants, i.e. alum, ferric chloride and a 1:1 mix of both. The optimum doses for coagulants were determined using a jar test. In stage-2, a nanofiltration (NF) membrane was used to further treat the effluent from stage-1. In CEPT, the 1:1 mixture of coagulants showed maximum removals, i.e. 75-77% for the total suspended solids and 73-75% for the chemical oxygen demand (COD). Stage-2 provided 85-95% removals for turbidity (0.88 nephelometric turbidity units), COD (41 mg/L), total dissolved solids (101 mg/L), hardness (11 mg/L as CaCO₃), chlorides (80 mg/L), and heavy metals (copper [0.03 mg/L] and lead [0.02 mg/L]). The operational time of the NF membrane was 46 min, 55 min and 70 min using alum, ferric chloride, and mix (1:1), respectively. Significant reduction was observed in membrane fouling for 1:1 mixture of coagulants. The effluent met the US Environmental Protection Agency guidelines for non-potable reuse.

A review on the integration of mainstream P-recovery strategies with enhanced biological phosphorus removal

Phosphorus (P), an essential nutrient for all organisms, urgently needs to be recovered due to the increasing demand and scarcity of this natural resource. Recovering P from wastewater is a feasible and promising way widely studied nowadays due to the need to remove P in wastewater treatment plants (WWTPs). When enhanced biological P removal (EBPR) is implemented, an innovative option is to recover P from the supernatant streams obtained in the mainstream water line, and then combine it with liquor-crystallisation recovery processes, being the final recovered product struvite, vivianite or hydroxyapatite. The basic idea of these mainstream P-recovery strategies is to take advantage of the ability of polyphosphate accumulating organisms (PAO) to increase P concentration under anaerobic conditions when some carbon source is available. This work shows the mainstream P-recovery technologies reported so far, both in continuous and sequenced batch reactors (SBR) based configurations. The amount of extraction, as a key parameter to balance the recovery efficiency and the maintenance of the EBPR of the system, should be the first design criterion. The maximum value of P-recovery efficiency for long-term operation with an adequate extraction ratio would be around 60%. Other relevant factors (e.g. COD/P ratio of the influent, need for an additional carbon source) and operational parameters (e.g. aeration, SRT, HRT) are also reported and discussed.

Nanocellulose for Sustainable Water Purification

Nanocelluloses (NC) are nature-based sustainable biomaterials, which not only possess cellulosic properties but also have the important hallmarks of nanomaterials, such as large surface area, versatile reactive sites or functionalities, and scaffolding stability to host inorganic nanoparticles. This class of nanomaterials offers new opportunities for a broad spectrum of applications for clean water production that were once thought impractical. This Review covers substantial discussions based on evaluative judgments of the recent literature and technical advancements in the fields of coagulation/flocculation, adsorption, photocatalysis, and membrane filtration for water decontamination through proper understanding of fundamental knowledge of NC, such as purity, crystallinity, surface chemistry and charge, suspension rheology, morphology, mechanical properties, and film stability. To supplement these, discussions on low-cost and scalable NC extraction, new characterizations including solution small-angle X-ray scattering evaluation, and structure-property relationships of NC are also reviewed. Identifying knowledge gaps and drawing perspectives could generate guidance to overcome uncertainties associated with the adaptation of NC-enabled water purification technologies. Furthermore, the topics of simultaneous removal of multipollutants disposal and proper handling of post/spent NC are discussed. We believe NC-enabled remediation nanomaterials can be integrated into a broad range of water treatments, greatly improving the cost-effectiveness and sustainability of water purification.

Electro-oxidation of tannery wastewater to achieve zero discharge - a step towards sustainability

Pilot-scale electro-oxidation equipment with a functional capacity of 0.2 m³/hr, with titanium electrodes coated with TiO₂/RuO₂/IrO₂ as both anodes and cathodes, was designed. It was installed on the premises of a commercial tannery. The waste streams from all the unit processes were combined. The composite wastewater, after conventional pre-treatment was subjected to electro-oxidation. The treated wastewater was reused four times with intermittent electro-oxidation treatment, after each reuse. EO could bring about a significant reduction in pollution load. Reduction in BOD, COD, TKN and TSS was 92%, 87.5%, 96.2% and 94.6% respectively. Generation of ^oOCl radicals, during electro-oxidation, were ascertained with DMPO-spin trapping techniques using Electron Spin Resonance (ESR) spectroscopy. The characteristics of the treated wastewater indicated that the wastewater was fit for reuse. No significant change in the quality of the water after each reuse was observed. The physical properties of the leathers obtained following the reuse processes were akin to those of the control leathers, which was indicative that the reuse did not cause adverse quality deviations. This technique could provide the plausibility for minizine the discharge of wastewater to near-zero level.

A Review on Coagulation/Flocculation in Dewatering of Coal Slurry

Coal slurry is an essential component of mining operations, accounting for more than half of operating costs. Dewatering technology is simultaneously confronted with obstacles and possibilities, and it may yet be improved as the crucial step for reducing the ultimate processing cost. Coagulation/flocculation is used as a dewatering process that is reasonably cost-effective and user-friendly. This paper reviews application of different coagulants/flocculants and their combinations in dewatering mechanisms. In this context, various polymeric flocculants are discussed in the coal slurry in depth. Many operational parameters that influence the performance of coal slurry flocculation are also presented. Furthermore, a discussion is provided on the mechanism of flocculants’ interaction, the strategy of combining flocculants, and efficient selection methods of flocculants. Finally, coagulation/flocculation remaining challenges and technological improvements for the better development of highly efficient treatment methods were highlighted, focusing on the intricate composition of slurry and its treatment difficulties.

Application of Natural Coagulants for Pharmaceutical Removal from Water and Wastewater: A Review

Pharmaceutical contamination threatens both humans and the environment, and several technologies have been adapted for the removal of pharmaceuticals. The coagulation-flocculation process demonstrates a feasible solution for pharmaceutical removal. However, the chemical coagulation process has its drawbacks, such as excessive and toxic sludge production and high production cost. To overcome these shortcomings, the feasibility of natural-based coagulants, due to their biodegradability, safety, and availability, has been investigated by several researchers. This review presented the recent advances of using natural coagulants for pharmaceutical compound removal from aqueous solutions. The main mechanisms of natural coagulants for pharmaceutical removal from water and wastewater are charge neutralization and polymer bridges. Natural coagulants extracted from plants are more commonly investigated than those extracted from animals due to their affordability. Natural coagulants are competitive in terms of their performance and environmental sustainability. Developing a reliable extraction method is required, and therefore further investigation is essential to obtain a complete insight regarding the performance and the effect of environmental factors during pharmaceutical removal by natural coagulants. Finally, the indirect application of natural coagulants is an essential step for implementing green water and wastewater treatment technologies.

Impacts of aluminum- and iron-based coagulants on municipal sludge anaerobic digestibility, dewaterability, and odor emission

Although aluminum- and iron-based chemicals have been broadly used as the two most common types of coagulants for wastewater treatment, their impacts on the performance of downstream sludge management can be quite different and have not been well understood. This work reviewed and analyzed their similarities and differences in the context of the anaerobic digestion performance, dewaterability of digested sludge, and odor emission from dewatered biosolids. In short, iron-based coagulants tend to show less negative impact than aluminum-based coagulants. This can be attributed to the reduction of ferric to ferrous ions in the course of anaerobic digestion, which leads to a suite of changes in protein bioavailability, alkalinity and hydrogen sulfide levels, and in turn the sludge dewaterability and odor potential. Whether these observations still hold true in the context of thermally hydrolyzed sludge management remains to be studied. PRACTITIONER POINTS: The impacts of aluminum-/iron-based coagulant addition on municipal sludge anaerobic digestibility, dewaterability, and odor emission are reviewed. Iron-based coagulants show less negative impact on the sludge digestibility than aluminum-based coagulants. Conclusions may aid practitioners in selecting coagulants in practice and better understanding the mechanisms behind the phenomena.

Biological nutrient removal enhancement using fermented primary and rotating belt filter biosolids

This research compared the impact of two primary treatment options (i.e. primary clarification and rotating belt filtration (RBF)) on biological nutrients removal (BNR) process, using sludge fermentation liquid (SFL) as a carbon source. The liquid fraction of both fermented primary and RBF sludges comparably enhanced BNR. Despite the significant contribution of the unpurified SFL to the sharp increase in nutrient levels; i.e. 47%-64% (primary effluent; PE), and 45%-53% (RBF) of the soluble nitrogen and phosphorus loads; readily biodegradable COD and volatile fatty acids (VFAs) fractions of the combined feed increased significantly (2.5-6.1 times), compared to the original feed by additional SFL. Removal efficiencies in the reactors reached 57% (total nitrogen) and 92% (total phosphorus) after addition of SFL. Effluent nitrogen and phosphorus of the two reactors were close in the range of 15 ± 6 mg N/L, and 0.5 ± 0.3 mg P/L, respectively. Kinetics studies showed denitrification rates of 1.3, and 1.13 kg NO₃-N/m³.d for primary effluent and RBF effluent-fed reactors, respectively. Phosphorus release rates were 11.7 and 9.7 mg PO₄-P/g VSS.h, for primary, and RBF effluents, respectively; showing 20%-22% lower rates in the RBF SFL. Incorporating experimental data into a plant-wide model for a 100 MLD facility receiving typical medium strength wastewater, showed that although primary treatment enhanced the biogas production by 96% (primary clarification) and 62% (RBF) trains; combined fermentation and anaerobic digestion was effective to enhance the biogas production by 59% on average, compared to the base scenario without primary treatment. Additionally, if primary clarification exists, then the addition of fermentation results in additional revenue of C$1890/d in the plant, considering additional revenue of C$2230/d due to VFA generation in contrast to only C$340/d loss due to the reduced methane production.

A machine learning framework to improve effluent quality control in wastewater treatment plants

Due to the intrinsic complexity of wastewater treatment plant (WWTP) processes, it is always challenging to respond promptly and appropriately to the dynamic process conditions in order to ensure the quality of the effluent, especially when operational cost is a major concern. Machine Learning (ML) methods have therefore been used to model WWTP processes in order to avoid various shortcomings of conventional mechanistic models. However, to the best of the authors' knowledge, no ML applications have focused on investigating how operational factors can affect effluent quality. Additionally, the time lags between process steps have always been neglected, making it difficult to explain the relationships between operational factors and effluent quality. Therefore, this paper presents a novel ML-based framework designed to improve effluent quality control in WWTPs by clarifying the relationships between operational variables and effluent parameters. The framework consists of Random Forest (RF) models, Deep Neural Network (DNN) models, Variable Importance Measure (VIM) analyses, and Partial Dependence Plot (PDP) analyses, and uses a novel approach to account for the impact of time lags between processes. Details of the framework are provided along with a demonstration of its practical applicability based on a case study of the Umeå WWTP in Sweden involving a large number of samples (105763) representing the full scale of the plant's operations. Two effluent parameters, Total Suspended Solids in effluent (TSS_e) and Phosphate in effluent (PO4_e), and thirty-two operational variables are studied. RF models are developed, validated using DNN models as references, and shown to be suitable for VIM and PDP analyses. VIM identifies the variables that most strongly influence TSS_e and PO4_e, while PDP elucidates their specific effects on TSS_e and PO4_e. The major findings are: (1) Influent temperature is the most influential variable for both TSS_e and PO4_e, but it affects them in different ways; (2) PO4_e depends strongly on the TSS in aeration basins - higher TSS concentrations in aeration basins generally promote PO₄ removal, but excess TSS can have negative effects; (3) In general, the impact of TSS in aeration basins on TSS_e and PO4_e increases with the distances of the basin from the merging outlet, so more attention should be paid to the TSS concentration in the third or fourth aeration basins than the first and second ones; (4) Returning excessive amounts of sludge through the second return sludge pipe should be avoided because of its adverse impact on TSS_e removal. These results could support the development of more advanced control strategies to increase control precision and reduce running costs in the Umeå WWTP and other similarly configured WWTPs. The framework could also be applied to other parameters in WWTPs and industrial processes in general if sufficient high-resolution data are available.